Notice the inlet into the spillway from the river on the right side of this picture.
Bonnet Carre' Spillway
Although it is possible to pass floods in excess of 20 feet on the Carrolton gage, such stages cause disruption of waterborne traffic, necessitate removal of vast quantities of commodities from wharves and river front storehouses, cause panic and perhaps evacuation of the city by a considerable percentage of the population...
-United States Corps of Engineers Office, 2nd District in New Orleans, 1937
Notice the inlet into the spillway from the river on the right side of this picture.
In 1927, as the waters of the "Great Flood" steadily rose, New Orleans, one of the busiest ports and most influential cities in the United States, cast a wary eye at the approaching floodwaters. Citizens of the town watched the river gauge level north of town at Carrolton, wondering if they would have to evacuate their families and leave their life's possessions behind. The business community of New Orleans looked upon the rising floodwaters and wondered if the city could financially survive another flood, especially one that was already devastating the entire alluvial valley north of the city.
As John Barry described in his best selling novel Rising Tide, the business leaders of New Orleans settled upon a plan of how to avert a potential disaster in New Orleans if the levees north of town couldn't hold. Their final solution was to blow up the levee south of town, at Caernarvon in St. Bernard Parish, to relieve the pressure that the rising waters were placing upon the levee system as a whole in and around Orleans Parish. While this action would save New Orleans and all of its wealth and commerce, it would also displace much of the population of St. Bernard and Plaquemines Parishes, washing away all of the holdings of the effected citizens. Resentment is still felt among those who were alive at the time of the dynamiting at Caernarvon in St. Bernard Parish to those who chose to sacrifice the homes of the less powerful to protect the homes of the wealthy. (Barry 256)
Dynamiting of levee at Caernarvon, 1927
When the Flood Control Act of May 1928 was being drafted, considerations were being put into place to prevent a repeat of artificial crevassing in order to save New Orleans from future floods. As the MR&T program began to take shape, the previous ideology of "levees only" was discarded for a more expansive plan of combining levees and outlet channels to route excess water flow away from the main channel of the Mississippi River to various other floodways to then empty into the Gulf of Mexico. This would relieve pressure on the levee system in and around New Orleans. The base for this ambitious plan was to be the Bonnet Carre' Spillway, located north of New Orleans. (USACE Web site, Bonnet Carre' Spillway: Site Selection) This site was chosen because it was the location of four previous levee crevasses between 1849 and 1882, denoting that it was an extremely problematic and sensitive area. Control of a flood control structure here would allow for the Corps of Engineers to take this highly sensitive area and use it as mother nature had intended, as a relief valve during high water. (Read 2) This is an example of how flood control projects can bow to the wishes of mother nature.
Almost immediately after the passage of the Flood Control Act of 1928, the Corps of Engineers began building the Bonnet Carre' Spillway. For it's time, the spillway was a design marvel, especially when it is taken into account that New Orleans, itself, is an engineer's worst nightmare. The fact that much of the area surrounding the city was extremely wet and marsh-like, compounded with the ever-present problem of land subsidence that has South Louisiana sinking into the Gulf of Mexico, made the design of the spillway structure critical. Interestingly, the Corps used a little old fashioned know-how combined with precedent establishing ingenuity to create the optimal structure on-site. The old-fashioned know-how came in the Corps' study of what type of pilings (and how deep they needed to go) to use. The Corps had to look no farther than an already established New Orleans monument, the statue of General Robert E. Lee in the middle of Lee Circle, to figure out that untreated wooden pilings were sufficient so long as they were driven down below the area's water table. While this study hinted to the Corps of Engineers resourcefulness, it was their use of new technology in regards to advances in concrete that made the spillway structure at Bonnet Carre' a truly remarkable accomplishment. Since they would need over 135,000 cubic yards of concrete of sufficient strength in order to fight the forces of the river and the settling of the land, the engineers established a lab to produce a perfectly mixed concrete to deal with the special forces that the structure would encounter. Previously, concrete was only able to handle a compressive force (basically, the weight each square inch could effectively hold) of roughly 3000 pounds per square inch. The Corps' lab designed a concrete that could handle just over 5000 pounds per square inch. Therefore, the Corps designed a much stronger concrete. (USACE web site, Bonnet Carre'' Spillway: Design Advances) The final product in 1931 has seen little if no changes from when it was built. This is a testament to the Corps' design advances. The cost of this project has been debated. Estimates have it between $13,266,000 and $14,212,200. It is incomprehensible to imagine what such a project would cost today, and it is equally incomprehensible to think how much money has been saved due to the success of this project.
Here, briefly are some of the logistics of the structure, itself. A further exploration of the technical details of the spillway can be found by clicking on the link at the bottom of the page. The following information was obtained from the book Bonnet Carre' Spillway: Operation 1937, p. 1.
· Begun in 1928. Finished in 1931.
· Can pass 250,000 cubic feet/second of water through its bays.
· 32.8 river miles north of New Orleans. Just north of Norco, LA.
· Spillway length= 5.7 miles from the river to Lake Ponchartrain
· Route of water travel= Mississippi River into Lake Ponchartrain into the Rigolets into the Gulf.
· Structure is 7000 feet long, composed of 300 bays. Bays are filled with pine "needles" that are lifted out by two cranes.
· Guide levees are roughly 19 feet high.
· Opened only eight times in its history: 1937, 1945, 1950, 1973, 1975, 1979, 1983, and 1997.
· Interestingly, the timber pins are constantly allowing some river water to escape into the floodway. The sediment that is deposited into the floodway (in this instance
it is not enough water to have a flow going all the way to the lake) is dug up and shipped off for various uses.
· The spillway is opened when the river gauge threatens to top 20 feet at Carrolton, just north of New Orleans. See the bureaucratic channels required to do so
below.
When there is a pressing need for the spillway to be opened, there are some bureaucratic channels that have to be exercised first. As the water level approaches twenty feet at the Carrolton gauge, the Colonel in charge of the Corps' New Orleans District must call the Mississippi River Commission President in Vicksburg, Mississippi. From there, the Commission President consults with the Chief of Engineers in Washington, D.C. If these men believe that the river is rising to such a level that might threaten New Orleans, they will authorize the opening of the spillway until the threat has passed. (Read 3)
Much debate has occurred over wether or not the openings of the Bonnet Carre' spillway have a positive or a negative effect upon Lake Ponchartrain and its marine life. Detractors of opening the spillway claim that the introduction of the freshwater from the Mississippi River has drastic effects upon the marine life that resides in the brackish lake. Algal blooms have been known to occur in the lake after the spillway has been opened, but there is very little proof that the opening of the spillway effects the fish and oysters of the lake. In fact, the Corps of Engineers proposes that the opposite will occur over time. Their stance is that the introduction of the freshwater will replenish the lake with vital nutrients that, in the long run, will help to strengthen the lake as a fishery.
Click on these links to learn more about the Bonnet Carre' Spillway
1.) http://www.mvn.usace.army.mil/pao/bcarre/bcarre.htm
2.) http://www.mvn.usace.army.mil/index.asp
Click on these links to see pictures of the Bonnet Carre' Spillway
1.) http://www.mvn.usace.army.mil/images.html